The present invention relates to an improved construction of apparatus for selectively moving, for instance rotating and/or displacing in axial direction a machine element.
Injection molding machines oftentimes possess a hollow cylinder equipped with a worm spindle which is rotatable and axially displaceable therein, by means of which the plastic-molding material is plasticized and subsequently expelled through a nozzle into the molding tool. The end of the worm spindle facing away from the nozzle is operatively coupled with a drive apparatus by means of which there can be alternately or simultaneously produced a rotational movement and an axial displacement.
The heretofore known drive apparatuses of this type embody a hydraulic cylinder by means of which there can be displaced in the lengthwise direction of the worm spindle a punch which is in alignment with such worm spindle. The punch is connected with a rotatably and displaceably mounted shaft which may be equipped with a pinion meshing with the pinion of an electric or hydraulic motor.
During the plastification operation the plasticized molding mass tends to collect in front of the nozzle. Consequently, during the time that the worm spindle rotates it is successively displaced away from the nozzle towards the rear, in other words in the direction of the drive apparatus. As a result considerable axial forces are transmitted to the shaft of the drive apparatus. With the state-of-the-art drive apparatuses such axial forces are taken-up by an axial bearing which generally is constructed as a roller bearing. With most of the prior art constructional manifestations this axial bearing must also transmit that force by means of which the worm spindle is pushed towards the front during the time that the molding mass is pressed into the molding tool or mold. This axial bearing must be capable of taking-up very large forces and hence it must possess a large and robust construction. Consequently, such bearing considerably increases the costs of the machine.